This invention relates to chopping type electrical converter power supplies suitable for converting an unregulated input voltage to a regulated DC output voltage at a desired level. This invention is most easily described as it relates to solid state DC to DC converters using the well known fly-back chopping technique where the chopping frequency is varied to hold the output voltage constant at different load levels, and where step-down transformers are used to obtain voltage reduction, and capacitive output filtering is used to achieve acceptable ripple voltage levels.
Solid state converters of this type are well suited for use at lower voltages. However, for operation from relatively high voltage DC sources in the range of 600 to 1200 volts DC, as is commonly supplied to power electric rail vehicles, the stress placed upon the converter operating components is extremely severe and results in frequent component failures. This is due in part to the extremely harsh electrical environment in which the converter must operate and in part to the inherent operating limitations of the components themselves.
Electric rail vehicles obtaining high voltage DC power from third rail or catenary via sliding collectors require a low level stable DC voltage suitable for providing lighting and control power and for recharging on board batteries. The high voltage input to a converter mounted in an electric subway or railway vehicle is subject to extreme variations in level, as the sliding collector of the vehicle in which the converter is mounted makes intermittent contact with the third rail or catenary, and as the intermittent contact of other vehicles imposes switching transients on the high voltage line. As a consequence, the electrical equipment which is connected directly to the high voltage input is subjected to frequent high energy and potentially destructive voltage transients.
Chopping-type designs using thyristors or silicon controlled rectifiers (SCR's) to chop the input voltage, followed by transformer voltage reduction and capacitive output filtering can efficiently convert a higher DC voltage to a lower one. However, when operated at high voltage levels, previous designs have required the SCR's and the electrolytic filter capacitors (which are commonly used due to their small physical size) to operate at or near their operative limits.
For example, it is known that an SCR chopping a given input voltage will encounter voltages across it of twice the input voltage even when the input voltage is well regulated. For 600 volt operation, previous designs using two chopping SCR's have required SCR's rated at 1,000 volts, and for a high power converter (above 5 kilowatts, for example) where the 600 volt line has the type of high energy transients previously described, even this rating is not adequate. Unfortunately, small, high speed, high power SCR's with a higher voltage rating such as are required to achieve a high chopping rate are not commonly available.
A second problem encountered in high voltage operation is that electrolytic capacitors are not readily available rated at voltages in excess of 450 volts. Furthermore, in order to obtain a reasonable operating lifetime from such capacitors, they should be operated at 60% or less of their rated voltage. Accordingly, for two capacitors in series functioning as an input filter, as used in the prior art design, 600 volts is the maximum permissible operating voltage for reliability purposes. This leaves little or no margin for handling voltage transients.
In order to operate at 600 volts, previous converter designs have relied upon expensive and bulky input filters specially designed to trap and absorb the high energy transients before they could reach and damage the SCR's and electrolytic capacitors. This invention is capable of continuously operating at twice the input voltage of previous designs and can eliminate the bulky transient filters previously required.
A second major problem encountered in previous designs has been the low operating frequency inherent in such designs. In chopper-type converters there exists a close relationship between the physical size of the converter and its operating frequency. The higher the frequency at which the converter can be operated, the smaller the filter capacitors and magnetic components can be. At low power levels, the primary limit on capacitor size is that the capacitor must be large enough to reduce voltage ripple to an acceptable level. The higher the operating frequency, the less the voltage ripple for a given capacitance.
At high power levels, an even more severe limit is placed on capacitor size, requiring a capacitor larger than would be required to satisfy the voltage ripple requirement. At such high power levels, filter capacitors are subject to very high ripple currents. In a capacitor chosen just large enough to satisfy the voltage ripple requirement, the high ripple current causes unacceptable I.sup.2 R heating of the capacitor and premature failure.
Operating at a higher frequency reduces both the ripple current and the ripple voltage allowing smaller filter capacitors to be utilized. Because filter capacitors are one of the larger converter components, this allows a substantial reduction in converter size. Unfortunately, as will be described in greater detail, previous designs have been limited in the maximum frequency at which they could operate. This invention is capable of operating at a frequency in excess of twice the frequency of previous designs.
Previous converter designs have used a voltage controlled oscillator to generate trigger pulses for the SRC's. As the load draws a heavy current, the output voltage tends to fall, and this falling voltage is sensed and used to increase the oscillator frequency to maintain the converter output voltage.
In a chopper-type design using step-down transformers, the maximum operating frequency is limited by two primary considerations. The first is that "backfiring" must not occur. Backfiring refers to applying a trigger pulse to an SCR located in the primary circuit of the step-down transformer before current has ceased to flow in the secondary circuit of the transformer, which can lead to very dangerous and destructive voltage transients.
The second consideration is that "crossfiring" must not occur. Crossfiring refers to applying a trigger pulse to the second SCR in a pair while current continues to flow through the first SCR. Crossfiring two SCR's leads to a direct short across their high voltage input.
In order to avoid crossfiring or backfiring, prior art designs have set a maximum operating frequency for the oscillator, selected in advance to be low enough to avoid these two problems in the "worst-case" which the converter might face. However, the converter seldom, if ever, must operate under worst case conditions, and under normal operating conditions, the worst-case limit on the operating frequency is substantially more restrictive than is necessary.
This invention avoids the use of such a worst-case upper frequency limit by continuously monitoring important circuit parameters and setting the upper frequency limit according to the instantaneous circuit operating conditions.
Previous designs have also suffered from other problems which this invention has solved. With some types of oscillating loads, the type of free-running oscillator with voltage control previously used could set up "harmonic" or self-oscillating modes of converter operation wherein essentially all of the converter load is placed on a single SCR causing it to fail. Additionally, non-linear operation of the type of free-running oscillator previously used has greatly complicated converter design.